DSL is a popular way to gain broadband access to the Internet. DSL's distinct advantage over its competitors is its ability to provide such broadband access using an ordinary, twisted-pair copper telephone line, commonly referred to as a “local loop.” DSL calls for a DSL modem to be located at a user's “premises” and a corresponding DSL Access Multiplexer (DSLAM) at the central office (CO) that serves the user.
DSL works by dividing the total frequency spectrum available over the telephone line into two bands: a narrow lower band consisting of frequencies of 25 kHz and below and a broad upper band consisting of frequencies above 25 kHz. The lower band is retained for conventional telephone use; the upper band provides the broadband access. DMT DSL calls for the upper band to be divided into a large number of narrowband subchannels, on the order of a few hundred subchannels to be more specific. Some of the subchannels are designated for downstream data and others for upstream data. If the number of downstream subchannels equals the number of upstream subchannels, the DSL is said to be symmetric; otherwise, it is said to be asymmetric.
DMT was selected for DSL due to its ability to react to interference. Interference occurs on a telephone line when an external event (such as lightning) creates static in the line or the signals being transmitted over the line interfere with themselves. Interference is rarely full-spectrum; instead it is often limited to a range of frequencies. While interference may impair some of the DMT subchannels, many if not most subchannels are likely to remain unaffected and available to carry signals.
DMT DSL requires that all of its subchannels be monitored for quality (typically expressed in terms of signal-to-noise ratio, or SNR). Then, an algorithm is used to allocate bits to be transmitted among the subchannels as a function of relative subchannel quality. The resulting allocation (or “loading”) scheme, taking into account any user-configurable preferences, is referred to as “bit loading.” Bit loading calls for more bits to be allocated to a DMT subchannel that has a higher SNR than it does to one that has a lower SNR. The DSL modem, which is at the user's premises, is responsible for measuring the SNR of the DMT subchannels, determining the bit loading and transmitting the bits accordingly. DMT DSL that dynamically changes subchannel loading is called “adaptive,” because bit transmission adapts to changing subchannel conditions.
Modern DSL framing parameters include the number of bits loaded on each DMT subchannel, Reed-Solomon coding redundancy and codeword size, whether trellis coding is enabled or disabled, interleaver depth and overhead subchannel rate. The user can configure the maximum delay, the minimum overhead rate, the maximum and minimum data rate (including specifying “best possible”), the minimum impulse noise protection, the nominal and maximum PSD level and the allowable subchannel set.
Unfortunately, no straightforward way exists to select the modem framing parameters to optimize globally for all input parameters. This problem is conventionally solved through an algorithm that searches over a variety of input parameter settings. For example, the algorithm might search over trellis code enabled and disabled, varying Reed-Solomon redundancy values, and a range of gross coding gains.
Subchannel coding gain is the effective increase in subchannel gain that results purely from the coding scheme applied to information transmitted over a given subchannel; gross coding gain is the sum of all subchannel coding gains. the higher the gross coding gain, the more bits can be loaded on each DMT subchannel. However, higher coding gains typically require more coding redundancy to achieve which may or may not result in a higher overall data rate (or more impulse noise protection or a higher or lower delay depending on the particular values of the user-configurable parameters).
What is needed in the art is a way to estimate modem framing parameters globally for at least a subset of input parameters so that the overall net data rate of a DMT DSL modem is increased. More specifically, what is needed in the art is a way to estimate modem framing parameters globally for all input parameters such that errors in estimation are reduced, and most advantageously minimized, over a wide range of operating conditions.